Many traffic signals already installed are of the simple fixed cycle variety provided by an electro-mechanical cam drive system. These systems have limited load capacity and have no provisions for flashing a pedestrian dont-walk signal on and off during a clearance period. It is a significant problem to rework such systems in the field in both time and expense to conform such equipment for the flashing mode of operation. Although flashers are known, the use of prior art techniques such as shown in U.S. Pat. Nos. 3,286,230 and 3,383,653, for example, would result in extensive system change and the adoption of complex interface equipment.
Switching relays used in producing strobing signals for flashing pedestrian control lamps are not reliable over long operating periods since they tend to break heavy a-c load currents and arc; and, furthermore, they need an external strobing mechanism.